This invention relates to a method for converting colors from a first device color space to gray levels for reproduction by a color or monochrome rendering device. More particularly, the invention relates to a method for attaining improved gray representations from a color or monochrome printer in response to an input color image.
It is well known that color monitors exhibit a larger color gamut than currently available laser and inkjet color printers. The prior art has suggested a number of methods for achieving more faithful reproduction of monitor colors from such printers. For instance, U.S. Pat. No. 5,081,529 to Collette describes a color calibration system that enables a user to achieve such more faithful color reproductions. Essentially, a hard copy print is made from a processed image and if it does not match the monitor image colors, a color correction algorithm is invoked which is responsive to operator adjustments to revise the color controls within the printer. Accordingly, after several iterations, a more faithful color reproduction from the printer is achieved.
The procedure described in the Collette patent can be termed a xe2x80x9cgamutxe2x80x9d mapping action wherein the colors that can be produced by one device are mapped into a color space that can be reproduced by another device. Morovic et al. in xe2x80x9cGamut Mapping Algorithms Based on Psychophysical Experimentxe2x80x9d, The Fifth Color Imaging Conference: Color Science, Systems and Applications, Nov. 17-20, 1997, pp 44-49, describe five gamut mapping algorithms. Morovic et al., in one example, derive an image""s red, green and blue (RGB) values that are utilized to present the image on a monitor. Those values are first transformed into XYZ tristimulus values, a transformation specified by a 1931 standard published by the CIE (Commission Internationale de L""Eclairage). The XYZ tristimulus values are then utilized to enable CIE L*a*b* coordinates to be calculated.
The CIE L*a*b*color space is a device independent color space that is produced by plotting, in rectangular coordinates, the L*a*b* quantities defined by the following equations:
L*=116(Y/Yn)⅓xe2x88x9216
where:
Y/Yn greater than 0.008856
a*=500{(X/Xn)⅓xe2x88x92(Y/Yn)⅓)}
xe2x80x83where:
X/Xn greater than 0.008856
Y/Yn greater than 0.008856
b*=200{(Y/Yn)⅓(Z/Zn)⅓)}
xe2x80x83where:
Z/Zn greater than 0.008856
Y/Yn greater than 0.008856
where Xn, Yn, Zn specifies the xe2x80x9cwhite object color stimulusxe2x80x9d. FIG. 1 illustrates the coordinate assignments in the CIE L*a*b* color space.
Morovic et al. then perform gamut mapping in CIE L*a*b* space and the resulting CIE L*a*b* coordinates are then transformed back to XYZ tristimulus coordinate values, using an output device""s substrate as the white point. The resulting tristimulus XYZ values are then transformed into colorant amounts for an ink jet printer.
The procedure utilized by Morovic et al. is schematically illustrated in FIG. 2, wherein the monitor RGB values are transformed to CIE L*a*b* color space which is then gamut mapped to the printer""s CIE L*a*b* color space. The resulting L*a*b* color values are then further mapped to the printer""s cyan, magenta and yellow primary colors. The gamut mapping, as achieved by the aforesaid conversion, is then utilized to create a color map which is installed in the printer.
To convert monitor RGB values to gray levels for reproduction by a color printer, the prior art (see FIG. 3) has converted the monitor RGB color values to CIE L*a*b* color values (via an intermediate XYZ conversion). Thereafter, the lightness (i.e., L*) values have been directly mapped to printer gray levels. Once such a gamut mapping was achieved, the resulting mapped values were converted to a table which was used to control the printer.
A more common prior art procedure for converting RGB colors to gray levels is described in SMPTE RECOMMENDED PRACTICE, RP 177-1993, section 3.3.8. In that document, the luminance value (i.e., the Y coordinate in XYZ color space) is defined as:
Y=YR(R)+YG(G)+YB(B)
The derived Y luminance values are then scaled to gray levels.
Gamut mapping from a monitor""s color space to a color or monochrome printer""s gray levels has presented certain special problems. For instance, the above indicated color conversion procedure results in certain color components not reproducing well in the resulting gray image. For example, certain cyan components and yellow components of the image will disappear. This is especially the case when light and medium green colors are fed from the monitor to the printerxe2x80x94as the printer is particularly adapted to producing dark greens and not the lighter shades. Accordingly, the lighter shades are reproduced poorly or not at all.
Accordingly, it is an object of this invention to provide an improved method for converting colors from a first device color space to gray levels which can be reproduced by a color or monochrome rendering device.
It is a further object of this invention to provide a method for converting colors from a first device color space to gray levels in a second device color space wherein improved gray representations are achieved.
The method of the invention converts colors resident in a first device color space to gray levels for reproduction in a color or monochrome rendering device. The method initially converts the color values resident in the first device color space to corresponding color values in a device independent color space. Next, the resulting color values in the device independent color space are converted to further color values in the same color space that correspond to color values resident in the color space of a target color rendering device. The further color values from the device independent color space are then converted to corresponding gray levels so that they may be rendered by either the target color rendering device or a different color or monochrome rendering device. The use of an intermediate gamut conversion within the device independent color space enables more precise gray representations to be achieved for certain color representations.